This invention relates to adaptive control of cutting operations on CNC-operated machine tools in which a controlled input parameter characterizing the movement of a cutting tool relative to a workpiece, is continuously adjusted during a cutting operation in response to a measured output operation parameter defining the productivity of the operation. The present invention particularly concerns the adaptive control of turning operations performed on lathes, where the controlled input parameter is a feed rate of the cutting tool and the output parameter is a cutting torque, cutting force or consumed power of the lathe""s spindle drive.
In a CNC-operated lathe, a program instructs a feeding means on a feed rate with which a tuning tool should cut a workpiece and instructs the lathe""s spindle drive on a speed with which a workpiece associated therewith should be rotated. The feed rate and the selected speed are controlled input parameters that are normally fixed by the program for each cutting operation based on pre-programmed cutting conditions such as depth of cut, diameter of the workpiece, material of the workpiece to be machined, type of the cutting tool, etc.
However, the efficiency of CNC programs is limited by their incapability to take into account unpredictable real-time changes of some of the cutting conditions, namely the changes of the depth of cut, non-uniformity of a workpiece material, tool wear, etc.
Optimization of cutting operations on CNC-operated lathes, as well as on most other machine tools, is usually associated with the adaptive control of the movement of a cutting tool relative to a workpiece and, particularly, with the adjustment of the cutting tool""s feed rate as a function of a measured cutting torque developed by the machine tool, to compensate the change in cutting conditions.
FIG. 3 illustrates a known control system for adaptively controlling a turning operation, for use with a CNC-operated lathe having a feeding means and a spindle drive that are instructed by a CNC program to establish the movement of, respectively, a cutting tool and a workpiece attached to the spindle, with pre-programmed values of respective controlled input parameters Fo that is a basic feed of the cutting tool and So that is a basic rotational speed of the spindle (the cutting tool and the workpiece are not shown).
As seen in FIG. 3, the control system comprises a torque sensor for measuring a cutting torque xcex94M developed by the spindle drive. Depending on an unpredictable variation of cutting conditions B, the cutting torque xcex94M may have different current values xcex94Mc, in accordance with which the torque sensor generates current signals Uc proportional to xcex94Mc. The control system also comprises a known adaptive controller including an amplifier with a signal transmission coefficient koxe2x80x2, transforming the signal Uc into koxe2x80x2Uc and subsequently determining a value Fo/Fo=ƒ(koxe2x80x2Uc) to which the feed rate Fc should be adjusted, by a feed rate override unit, in order to compensate the variation of the cutting conditions B and to, thereby, maintain the cutting torque xcex94Mc as close as possible to its maximal value xcex94Mmax, required for the maximal metal-working productivity.
The maximal value of the cutting torque xcex94Mmax is a predetermined cutting torque developed by the spindle drive during cutting with a maximal depth of cut, and the signal transmission coefficient of the amplifier is defined as             k      o      xe2x80x2        =          1              U        max              ,
where Umax is a signal from the torque sensor corresponding to the maximal torque xcex94Mmax.
The current value Fo/Fo is defined by the adaptive controller based on its signal transmission coefficient koxe2x80x2, pre-programmed basic feed rate Fo and signal Uc, in accordance with the following relationship:                                                         F              c                                      F              o                                =                      A            -                                          k                o                xe2x80x2                            ⁢                              U                c                                                    ,                            (        1        )            
where A=Fid/Fo, and Fid is an idle feed (feed without cutting).
The coefficient A characterizes the extent to which the feed rate Fc may be increased relative to its pre-programmed value Fo, and it usually does not exceed 2.
Since, as mentioned above, the signal Uc is proportional to the cutting torque xcex94Mc, the relationship (1) may be presented, for the purpose of explaining the physical model of the adaptive controller, as follows:                                                         F              c                                      F              o                                =                                    A              -                                                K                  o                  xe2x80x2                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                                  M                  c                                                      =                          a              c                                      ,                            (        2        )            
where Koxe2x80x2 is a correction coefficient corresponding to the signal transmission coefficient koxe2x80x2 of the adaptive controller and it is accordingly calculated as       K    o    xe2x80x2    =            1              Δ        ⁢                  xe2x80x83                ⁢                  M          max                      .  
The physical model of the adaptive controller is illustrated in FIG. 4. As seen, the change of the cutting conditions B influences the current value xcex94Mc of the cutting torque which is used by the adaptive controller to determine the coefficient ac characterizing the current value Fc to which the feed rate should be adjusted to compensate the changed cutting conditions B.
It is known that, in a turning operation, the cutting condition that changes unpredictably in time and that is mostly responsible for the variation of the cutting torque is the depth of cut hc=hc(t). When turning a workpiece of a given diameter, the cutting torque xcex94Mc is proportional to the depth of cut hc as follows:
xcex94Mc=cFchc=cFoachc, xe2x80x83xe2x80x83(3)
where c is a static coefficient established for turning operations and ac is defined in the equation (2).
Based on the equations (3) and (2), the cutting torque xcex94Mc may be expressed as:                               Δ          ⁢                      xe2x80x83                    ⁢                      M            c                          =                              A            ⁢                          xe2x80x83                        ⁢            c            ⁢                          xe2x80x83                        ⁢                          F              o                        ⁢                          h              c                                            1            +                          c              ⁢                              xe2x80x83                            ⁢                              F                o                            ⁢                              h                c                            ⁢                              K                o                xe2x80x2                                                                        (        4        )            
If in the equation (4), the coefficient A=2 and hc=hmax, the maximal cutting torque xcex94Mc may be expressed as:                               Δ          ⁢                      xe2x80x83                    ⁢                      M            max                          =                              2            ⁢                          xe2x80x83                        ⁢            c            ⁢                          xe2x80x83                        ⁢                          F              o                        ⁢                          h              max                                            1            +                                          cF                o                            ⁢                              h                max                            ⁢                              K                o                xe2x80x2                                                                        (        5        )            
Similarly, when the depth of cut is of a very small value hmin such that hmin/hmax less than  less than 1, the cutting torque xcex94Mmin will also be very small:
xcex94Mmin≈2cFohmin less than  less than xcex94Mmax xe2x80x83xe2x80x83(6)
It follows from the above that, with Me adaptive controller as described, there still may be a significant variation of the cutting torque xcex94Mc during cutting with the depth of cut varying in a wide range, as illustrated in FIG. 5, curve I.
It is the object of the present invention to provide a new method and system for the adaptive control of a turning operation.
In accordance with one aspect of the present invention there is provided a method of adaptively controlling a turning operation performed on a workpiece by a turning tool, by controlling an adjustable input operation parameter F of the movement of the turning tool relative to the workpiece, to maintain an output operation parameter xcex94M substantially at a predetermined value xcex94Mo and thereby to substantially compensate the variation of said output operation parameter xcex94M caused by the variation of at least one operation condition B=B(t) varying in time, the method comprising the steps of:
(a) measuring a current value xcex94Mc of the output parameter xcex94M,
(b) estimating the relation between xcex94Mc and xcex94Mo by multiplying xcex94Mc by a correction coefficient K which comprises an invariant correction coefficient component Ko inversely proportional to xcex94Mo, and
(c) determining a value Fc to which the input operation parameter F should be adjusted, as a function of Kxcex94Mc; characterized in that
(d) said correction coefficient K comprises a varying correction coefficient component whose current value Kc changes in accordance with the variation of said operation condition B=B(t), the step (b) further comprising calculating the current value Kc and calculating K=ƒ(Ko, Kc).
Preferably, K=Koxe2x88x92Kc.
The operation input parameter F is preferably a feed rate of the turning tool and the operation output parameter xcex94M is preferably a cutting torque developed by a drive rotating the workpiece. However, the operation output parameter may also be a cutting force applied by the tool to the workpiece or a power consumed by the drive.
The predetermined value xcex94Mo of the output parameter is preferably a maximal value xcex94Mmax which this parameter may have when the varying operation condition B differs to a maximal extent from its original or nominal value.
In accordance with preferred embodiments of the present invention, the invariant correction coefficient component Ko is defined as             K      o        =          A              Δ        ⁢                  xe2x80x83                ⁢                  M          max                      ,
where       A    =                  F        id                    F        o              ,
with Fid being an idle feed and Fo being a pre-programmed basic feed rate.
The varying operation condition B may be a real physical parameter such as a depth of cut hc=hc(t), hardness of the workpiece material, etc., whereby current values of the varying coefficient component Kc may then be obtained based on sensing current values of this parameter. Alternatively, the varying operation condition B may be a mathematical equivalent of one or more physical parameters of the cutting process.
In accordance with another aspect of the present invention, there is provided an adaptive control system for adaptively controlling a turning operation performed at a workpiece by a turning tool, by adjusting a controlled input operation parameter F to maintain an output operation parameter xcex94M substantially at a predetermined value xcex94Mo and thereby to substantially compensate variation of said output operation parameter xcex94M caused by the variation of at least one operation condition B=B(t), the system comprising:
a sensor of the output operation parameter xcex94M for providing a signal Uc proportional to a current value xcex94Mc;
an adaptive controller for determining a value Fc to which the input operation parameter F should be adjusted, as a function of kUc, where k is a signal transmission coefficient which comprises an invariant signal transmission coefficient component ko inversely proportional to xcex94Mo, said controller including an amplifier capable of transforming the signal Uc into kUc; and
an input parameter override unit capable of being controlled by said adaptive controller to adjust the controlled operation input parameter to Fc;
characterized in that
said controller further comprises a correction processing means for calculating kcUcc, where kc is a varying signal transmission coefficient component whose current values depend on the variation of said operation condition B=B(t), the controller being capable of calculating k=ƒ(ko,Kc).
Preferably, the adaptive controller is capable of calculating k=koxe2x88x92kc, and calculating ko as             k      o        =          A              U        o              ,
where Uo is a signal from the sensor of the operation output parameter corresponding to the value xcex94Mo. Preferably, xcex94Mo=xcex94Mmax and Uo=Umax.
Preferably, the sensor of the output operation parameter xcex94M is a sensor of a cutting torque developed by a drive rotating the workpiece and the input parameter override unit is a feed rate override unit.
The correction processing means may comprise a sensor or a calculator for, respectively, sensing or calculating current values of the operation condition B, to be subsequently used in the calculation of kc.